The standard deviation (this term was first used by Pearson, 1894) is a commonly used measure of variation. The standard deviation of a population of values is computed as:

s = [S(xi-m)2/N]1/2

where

m |
is the population mean |

N |
is the population size |

The sample estimate of the population standard deviation is computed as:

s = [S(xi-x-bar)2/(n-1)]1/2

where

x-bar |
is the sample mean |

n |
is the sample size |

See also, Descriptive Statistics Overview.